1. Field of the Invention
The present invention relates to the use of a specific class of sterically hindered amino compounds in admixture with a specific class of amino acids as co-promoters for alkaline salts in "hot pot" type acid gas scrubbing processes.
2. Description of Related Patents
Recently, it was shown in U.S. Pat. No. 4,112,050 that sterically hindered amines are superior to diethanolamine (DEA) as promoters for alkaline salts in the "hot pot" acid gas scrubbing process. U.S. Pat. No. 4,094,957 describes an improvement to this process whereby amino acids, especially sterically hindered amino acids, serve to prevent phase separation of the aqueous solution containing sterically hindered amines at high temperatures and low fractional conversions during the acid gas scrubbing process.
One of the preferred sterically hindered amines described in these patents is N-cyclohexyl 1,3-propanediamine. The bulky cyclohexane ring on this diamino compound provides steric hindrance to the carbamate formed at this site thereby favoring the expulsion of CO.sub.2 during regeneration, thereby leaving the hindered amino group free to protonate. The primary amino group of this diamino compound assists in maintaining solubility under lean conditions. Under lean conditions when there is insufficient carbonic acid present to protonate the hindered amino group, the molecule would be insoluble were it not for the primary amino group which forms a stable polar carbamate ion. However, even the carbamated primary amino group is insufficient to prevent insolubility of the compound under very lean conditions and an additional additive, as proposed in U.S. Pat. No. 4,094,957, an amino acid, is required to maintain solubility of the diamino compound. This amino acid also contributes to additional capacity and faster absorption rates for carbon dioxide, so it therefore acts as a co-promoter in addition to solubilizing the sterically hindered diamino compound. Screening studies of available amino acids as possible co-promoters for N-cyclohexyl 1,3-propanediamine based on cyclic capacity and rates of absorption ascertained that pipecolinic acid was one of the best amino acid co-promoters.
Pipecolinic acid, however, has shortcomings, e.g., it is rather expensive and its picoline precursor is in limited supply.
In view of the commercial potential of using the sterically hindered amino compounds as described and claimed in U.S. Pat. Nos. 4,094,957 and 4,112,050, there is a need for finding a sterically hindered amino compound promoter system which performs as well as the N-cyclohexyl-1,3-propanediamine-pipecolinic acid mixture. Specifically, there is a need for finding a less costly replacement for pipecolinic acid which also possesses its effectiveness.
Various amino acids have been proposed as promoters for alkaline salts in the "hot pot" gas scrubbing process. For example, British Pat. No. 1,305,718 describes the use of beta and gamma amino acids as promoters for alkaline salts in the "hot pot" acid gas treating process. These amino acids, however, are not suitable because the beta-amino acids undergo deamination when heated in aqueous potassium carbonate solutions. The gamma amino acids form insoluble lactams under the same conditions. Also, the alpha-amino acid, N-cyclohexyl glycine, as described in Belgian Pat. No. 767,105, forms an insoluble diketopiperazine when heated in aqueous solutions containing potassium carbonate.